The IEEE 802.16 standard specifies the air interface including Medium Access Control (MAC) and various specifications of the physical layer of a broadband wireless access (BWA) system for supporting various services. The IEEE 802.16 standard specifies two operating modes, namely a multipoint operating mode (PMP: point-to-multipoint mode) and an optional mesh operating mode.
In PMP mode, it is only possible for subscriber stations to communicate with a base station. The individual subscriber stations, for example mobile terminals, cannot communicate with one another directly. The individual subscriber stations must therefore be within the direct transmission range of the base station in order to obtain access to the network.
The other operating mode specified in the 802.16 standard is referred to as mesh or MSH mode. The main difference between PMP mode and MSH mode is that in PMP mode, the data is exchanged exclusively between the base station and the subscriber stations and no data can be transmitted directly between subscriber stations. In mesh mode, data can be exchanged directly between a subscriber station and the base station by routing the data via other subscriber stations. Data can also be exchanged directly between different subscriber stations. The advantage of mesh mode is that it permits indirect data communication between a subscriber device and the base station, i.e. it is possible to provide fewer base stations when implementing the network. Another advantage is that the base stations have a greater range extending beyond the physical broadcast range of their own network adapter.
The mesh data networks based on the 802.16-2004 standard do not describe any mechanism for broadcasting or multicasting data. In the case of a broadcast data transmission, a transmitting node sends data to all (broadcast) or to a specific subgroup of neighboring nodes (multicast). In order for a transmitting node to be able to send data to the neighboring nodes, according to the 802.16-2004 standard it must send its broadcast messages sequentially, i.e. the transmitting node sends the data as a unicast initially to a first receiving node and then subsequently to a further receiving node etc. until all neighboring nodes have been reached. The bandwidth required for this increases in line with the number of neighboring nodes. The time delay of the transmitted broadcast messages also rises. The time delay for subsequent messages likewise increases as a result of the effort expended for transmitting the broadcast message.
Distributed scheduling as used in mesh mode allows the node to coordinate its data transmissions in a distributed manner in its two-hop neighborhood, i.e. with nodes that are no further than two transmission hops or links away from the node. This enables data to be transferred freely without collisions or bottlenecks. However, the mesh mode defined in the IEEE 802.16 standard only supports requesting and granting of one bandwidth for a unicast data transmission, that is to say data transmission in which the data is received by exactly one neighboring node. The IEEE 802.16 standard does not support broadcast data transmission for mesh mode, that is to say successful transmission of a single data unit to multiple receiving nodes. For a successful broadcast data transmission, with the IEEE 802.16 standard a transmitting node would have to perform a data transmission reservation in a time slot in which all its neighboring nodes are ready to receive. The readiness of neighboring nodes to receive depends however on the reserved data transmissions in the neighborhood of the respective neighboring node. It is therefore not possible to base the selection of a time slot for data transmission solely on the knowledge of the state of the neighboring nodes of the transmitting node that wishes to perform a broadcast data transmission. In order to send data to all its neighboring nodes therefore, the transmitting node must sequentially send the data to its neighboring nodes in a unicast data transmission.
It is often necessary to transmit the same data to multiple receivers. For instance, in the case of reactive routing methods, protocol messages are often sent to multiple neighbors in order to determine a path to a destination. In this way the messages rapidly propagate throughout the entire network and the destination can be found more quickly. Using the conventional form of unicast data transmission has the disadvantage here that the data has to be transmitted additionally for each receiver. This is however very inefficient, as the same data is always transmitted. With a broadcast method on the other hand, one data transmission could reach multiple receivers simultaneously. Unfortunately, 802.16 does not currently provide any support for broadcasting data packets. There is one broadcast, but it is intended solely for transmitting MAC management in control subframe messages. This mechanism is not suitable for the actual data traffic. It controls the sending of data packets to multiple receivers, guaranteeing collision-free transmission.